1. Field of the Invention
The invention relates to improving the function of sensory cells. More specifically, this invention relates to a method and apparatus for effectively lowering the threshold of a sensory cell by inputting a noise signal to a sensory cell.
2. Background of the Invention
The nervous system can be divided into two main parts: the autonomic portion and the sensory portion. As used herein, the term autonomic refers to that portion of the nervous system that controls the functions of the body, such as, pumping blood, and perspiring, which occur involuntarily. The term sensory, on the other hand, refers to the systems of the nervous system which controls interaction of the body with its surroundings. Examples of sensory systems include the proprioceptive system, the auditory system, the visual system, the vibration-sensation system, the temperature-sensation system and the touch-pressure sensation system.
The sensory systems can be divided into two types of cells: sensory transducers and sensory neurons. Sensory transducers are cells such as the rod and cone cells in the visual system and the hair cells of the auditory system which interact with the surroundings. Sensory transducers convert an input signal, for example, a sound wave, into an electrical signal which the nervous system can process. Sensory neurons are cells which convey signals produced at the sensory transducers through the nervous system. Sensory transducers can actually be thought of as specialized neurons. Therefore, the term sensory cell will be used to describe both sensory neurons and sensory transducers.
A sensory cell typically includes a cell body or soma and one or more long processes: a single axon and dendrites. The single axon functions to carry signals from the soma while the dendrites carry signals to the soma. These processes act as cables conveying signals through the nervous system. Processes, however, are poorly insulated (which results in excessive leakage current) and are covered with a capacitive membrane (which gives rise to a propagation delay and signal decay over time). Therefore, processes are not good conductors and do. not rely on passive transmission to conduct a signal. Rather, processes convey nerve impulses through an active electrochemical mechanism called an action potential.
When a process is at equilibrium, there is a potential difference maintained across the membrane of a process due to differing ion concentrations (for example Na.sup.+, K.sup.+, Cl.sup.- and Ca.sup.2+) on either side of the membrane with the area within the membrane being at a lower potential than the area on the outside of the membrane. The ions chiefly responsible for conveying signals through processes are Na.sup.+ and K.sup.+. As is in most cells, a gradient is maintained in the concentration of these two ions by a Na.sup.- --K.sup.+ pump. The Na.sup.- --K.sup.+ pump maintains the concentration of Na.sup.+ about 9 times lower inside the process than outside the process and the concentration of K.sup.+ about 20 times higher inside the process than outside the process. The membrane, however, contains channels through which Na.sup.+ and K.sup.+ may pass. These channels are closed at equilibrium, but they open in response to a disturbance giving rise to a voltage. That is, they are voltage-gated.
An action potential is triggered (and a sensory cell "fires") when a disturbance of sufficient magnitude causes a localized depolarization of the membrane of a first magnitude sufficient to open the voltage-gated Na.sup.+ channels. The opening of the voltage-gated Na.sup.+ channels causes an influx of Na.sup.+ which leads to depolarization in adjacent areas of the process. The further depolarization leads to voltage-gated Na.sup.+ channels opening in the adjacent areas thereby causing the disturbance to propagate through the process. Equilibrium is restored, by, among other things, the opening of the voltage gated K.sup.+ channels when the depolarization reaches a second magnitude. When the voltage gated K.sup.+ channels open there is an outflux of K.sup.+ ions leading to a decrease in the magnitude of depolarization thereby causing the voltage-gated Na.sup.+ channels to close.
Sensory cells are typically threshold-based units. That is, if an initial disturbance to a cell is of insufficient magnitude, the resulting depolarization will be insufficient to open the voltage gated channels and will thus dissipate due to leakage current and the propagation delay caused by membrane capacitance. Such a disturbance is called a subthreshold signal.
Various diseases, disorders and injuries may increase the threshold of sensory cells. Such an increase in threshold may result in disturbances which are sufficient. to cause a healthy sensory cell to "fire" being insufficient to cause the damaged sensory cell to fire. Thus, any damage the body sustains to any of the transducer cells of the sensory systems, for example, may lead to the consistent occurrence of subthreshold input signals. Also, genetic defects, for example, visual and auditory impairment, may lead to the consistent production of subthreshold input signals to sensory cells.